The present invention concerns a method in the drying group or groups provided with a single-wire draw in a multi-cylinder dryer of a paper machine, for securing the threading of a web, the multi-cylinder dryer having drying cylinders heated by means of steam or equivalent. The web to be dried is pressed by the drying wire into direct contact with the cylinder faces of the drying cylinders. The multi-cylinder dryer also comprises leading cylinders or rolls, on which the web remains outside of the drying wire.
The present invention also concerns a device used in carrying out the above method, this device being applied in the area of a single-wire draw in a paper machine. The dryer comprises a line of heated drying cylinders, preferably upper cylinders, and a line of leading cylinders or rolls, with the web running between these lines by being supported while a drying wire so that the web is, on the heated drying cylinders, pressed by the drying wire into direct contact with the heated face of the drying cylinders, while on the leading cylinders or rolls the web is situated on an outer face of the drying wire. The leading cylinders or rolls are provided with outside grooves.
Furthermore, the device comprises arrangements situated in spaces between the drying cylinders and operating against free sectors of the leading cylinders, by means of which a reduced pressure level is maintained in the grooves. This reduced pressure level promotes adherence of the web to the drying wire on the sector of the outside curve.
It is frequently a problem with threading of a paper web in the area of a single-wire draw in the drying section of a paper machine, that on the lower cylinder or roll the leader is detached from the drying web, whereby the leader tends to move around and to be driven out of the machine.
In a manner known in the prior art, attempts have been made to eliminate this problem by increasing the difference of draw of the web out of these groups. Adjusting of the difference of draw to the correct level requires high precision, since an excessively high difference of draw cuts off the leader, and the attempt of threading must be restarted from the beginning. An increased difference of draw at the initial end of the machine reduces the strength of the paper at the final end of the machine, because the paper loses its potential of extension, and its elongation at rupture and its breaking energy become lower. This fact causes problems in the final end of the machine during threading.
In the area of the single-wire draw, when threading by means of ropes is employed, it is a further problem that the leader must be transferred in the lateral direction outwardly to the area of the ropes. This transfer requires devices of its own, which must be placed in a congested space. Moreover, the drying wire of the single-wire draw makes this lateral transfer more difficult.
With respect to the prior art related to the present invention, reference is made to the following patents or literature:
FI Pat. Appl. No. 771056 (Valmet Oy); PA1 WO 83/00514 (J. M. Voith GmbH); PA1 FI Pat. Appl. No. 862413 (Valmet Oy); and PA1 FI Pat. Appl. No. 873812 (Valmet Paper Machinery, Inc.).